The use of enzyme immunoassays (EIAs) to determine whether a particular analyte is present in a patient sample has aided the expansion of diagnostic medicine. In a typical enzyme immunoassay, an assay reagent is labeled with an enzyme, the reagent becoming bound to a solid support in an amount that depends upon the amount of analyte in the sample. Enzyme substrate, generally added in a final step of the assay, reacts with the enzyme to generate a detectable signal which is related to the amount of analyte present in the sample.
However, present in some patient samples are compounds which may interfere with enzyme activity. One form of interference occurs when an interfering compound acts to catalyze a signal generating reaction in the absence of the specific enzyme. The presence of such compounds can result in incorrect assay interpretation owing to the generation of signal in an amount unrelated to the presence of the analyte in the patient sample.
Blood or blood products (e.g., the contents of lysed red blood cells) are common sources of assay interference in enzyme assays which employ peroxidase as the enzyme label. The blood or blood products help convert the enzyme substrate to its oxidized products, giving rise to positive assay interference. Specifically, the heme component of hemoglobin can bind to solid supports used in EIAs to immobilize reagents. The heme moiety (also called "microperoxidase") present during signal development in peroxidase-catalyzed EIAs generates non-specific signal (i.e. color development) and is a source of positive interference.